Vacuum-bagging, or vacuum bag laminating as it is also known, is a technique widely used in the aircraft and aerospace industries to fabricate high strength-to-weight ratio fibre-reinforced composite components. Indeed, efforts are being made in aircraft construction to employ components on an increasing scale that consist either completely or at least partly of fibre-reinforced composite components, e.g. carbon-fibre-reinforced plastics (CFRP), as structural or load-bearing elements.
Vacuum-bagging is performed by sealing a plastic film or bag over or around a composite component, which may be positioned on a moulding tool. The composite component may, for example, be in the form of a wet laid-up laminate, or may comprise a core and/or reinforcing fibres into which a liquid polymer or resin is infused. The air enclosed by the sealing film or bag is then extracted or evacuated via a vacuum pump so that up to one atmosphere (1 bar) of pressure can be applied to the component. The compression generated by the vacuum thus helps to remove air, voids, and excess gas given off during curing in and around the component. To get optimal properties from carbon-fibre-reinforced plastic components made by hand lay-up or wet lay-up, most must be cured by a combination of heat and pressure (e.g. vacuum).
The vacuum bagging process is generally only as good as the method for detecting leakages in the sealing film or vacuum bag. Leakages can lead to porosities in the component and are often difficult to detect and to locate, particularly with large composite components. The methods usually employed are based on acoustic principles, but for small leakages and in a noisy environment and/or for large components, these methods are not very accurate, are unreliable and are difficult to automate.
To check the air-tightness of a sealing film or vacuum bag in a vacuum bagging assembly, a pressure loss inside the bag is measured over a specific time. If the pressure loss is higher than a predefined value, the seal is deemed inadequate (i.e. not tight) and a time- and labour-intensive search for the leakage starts. The sealing areas between the film or bag and the moulding tool are usually checked first and the entire tape joint forming the seal is compressed by hand to close any possible air gaps. If another measurement shows that this was not successful, a very sensitive microphone is used to check along the seal for fizzling noises. This requires quite a lot of experience and is difficult in a noisy environment. If the tape joint itself seems to be tight, then the entire sealing film or vacuum bag needs to be checked. This often simply results in a complete exchange of the sealing film or vacuum bag, however, as an inspection is so time- and labour-intensive that replacing the film or bag is often deemed to be a more economical approach.